The fuel cell is capable of producing electricity without a substantial need for fossil fuel that poses concerns about resource depletion, without noise, and at a high energy recovery rate as compared with other energy-based power generating systems. Great efforts have been made to exploit the fuel cell as a power generating plant of relatively compact size in buildings and factories, with some cells having been commercially implemented. In particular, polymer electrolyte fuel cells (PEFC) can operate at lower temperature than fuel cells of other types. The PEFC then draws attention not only as a device for household co-generation, but also as the replacement power source for internal combustion engines on vehicles because of the minimized corrosion concern regarding the materials of which cell components are made and their ability to discharge relatively high current flow despite low temperature operation. The PEFC is constructed of electrolyte membranes, separators and other components. The separator is generally a plate which is provided with a plurality of juxtaposed channels on one surface or both surfaces. The separator plays the role of conducting the electricity produced at the gas diffusion electrode within the fuel cell to the exterior, discharging water produced within the channels in the course of electricity generation, and securing the channels as a flow path for incoming reaction gas to the fuel cell. Such a fuel cell separator is required to be more compact in size. Since a multiplicity of separators are used in stack, there is a demand for a separator seal material having durability and long term service.
As the separator sealing material, packing materials based on various resins have been under study. Among them, sealing materials based on silicone rubber are often used for their moldability, heat resistance and elasticity. JP-A 11-129396 and JP-A 11-309747 disclose silicone rubber compositions of the addition cure type which are more effectively moldable to form seals. However, silicone rubbers obtained by curing these compositions are still unsatisfactory in maintaining elasticity over a long term.
In particular, packing materials for fuel cell separators are difficult to meet both the requirements of acid resistance and seal performance (or compression set) in acidic aqueous solution. JP-A 2002-309092 discloses the use of silicone resins as a solution to this problem. For seals on PEFC separators, however, not only acid resistance and low compression set, but adhesion to separator substrates is also an important factor. JP-A 2004-014150 discloses as a carbon separator gasket material a silicone rubber having minimized compression set in long-life coolant (LLC), but refers nowhere to the adherence of this silicone rubber. A primer suitable for use in fuel cell separators is disclosed in JP-A 2007-146147 where the rubber material is described merely as belonging to the addition cure type, but no further detail is described. Similarly, a primer suited for bonding a sealing material to a carbon separator substrate is disclosed in JP-A 2004-103290 where the rubber material is described as comprising preferably addition cure type liquid silicone, with no further detail.